Calcium phosphates are now widely used as food additives, dental materials, for artificial bones, etc.
In the production of calcium phosphates, a wet method in which calcium hydroxide and phosphoric acid are reacted in an aqueous fluid is generally employed. In accordance with the wet method, calcium hydroxide or calcium oxide is added to water to prepare a suspension (hereinafter referred to as "lime milk"), and an aqueous phosphoric acid solution is added thereto dropwise and reacted with the lime milk to precipitate a calcium phosphate.
In the wet method, however, calcium hydroxide is present chiefly in solid form in the lime milk because the solubility of calcium hydroxide in water is extremely low (solubility at 20.degree. C.: 0.126 g per 100 g of water). For this reason, even if phosphoric acid is added with vigorous stirring, it is difficult to consistently obtain a high quality product under the influences of granularity, form, and the like of solid calcium hydroxide. Moreover, during the storage of the lime milk, precipitation and solidification occur unless vigorous stirring is continuously applied For example, in the conventional wet method, since both calcium hydroxide in solid form and that in dissolved form react with phosphoric acid, unhomogeneous calcium phosphate having different diameters and properties is obtained.
Another problem is that in transporting the lime milk by means of e.g., a pump, adhesion to the pipe and solidification occur.
Thus, there has been proposed a method in which a water-soluble calcium salt such as calcium chloride or calcium nitrate, is used in place of the lime milk and is reacted with sodium phosphate, ammonium phosphate and the like in an aqueous solution to produce a calcium phosphate.
This method, however, has the disadvantage that undesirable compounds are produced as well as the desired calcium phosphate, requiring an additional step for removing the byproducts and complicating the process.